Helium fluid refrigerator



Jan. 2, 1,968 c. E. Wl TTER HELIUM FLUID REFRIGERATOR 2 Sheets-Sheet 1Filed Dec. 21, 1966 TR WY Wm R Ev 6m m W Y Jan. 2, 1968 c. E. WITTER3,360,955

HELIUM FLUID REFRIGERATOR Filed Dec. 21, 1966 2 Sheet-Sheet 2 CARROLLE.WITTER INVENTOR United States Patent Filed Dec. 21, 1966, Ser. No.605,127 9 Claims. (Cl. 62-335) This application is acontinuation-in-part of application Ser. No. 481,514, filed Aug. 23,1965, now abandoned.

This invention relates to a helium fluid refrigerator, and morespecifically to a refrigerator employing a closed circuitcompression-heat extraction-expansion flow path for producing cryogenicrefrigeration in the temperature range of below 20 K.

The advantages of providing operating environments in the temperatureregion below 20 K. for low electrical noise levels in heat loads such asmasers and other amplifiers for radio telescopes, communicationssatellites, military radar, superconducting computers, and otherelectronic equipment are now generally recognized.

The prior art has proposed and employed various types of helium fluidrefrigerators, one variety being a singlewalled casing construction witha high vacuum on the inner working volume of this casing for minimizingambient heat inleak. The cold components of the refrigerator arecontained within the high vacuum atmosphere, and there are severalresultant limitations. Any leakage or outgasing of the refrigerator coldcomponents or the heat load tends to destroy the thermal insulatingeffectiveness, resulting in inefiicient, erratic performance and thermalinstability. Another disadvantage is that the single walled casing is atthe warm ambient temperature throughout its entire length, thus causinghigh radiative heat transfer losses to the cold end of the refrigerator.That is, the temperature difference between the adjacent single walledcasing and the enclosed refrigerator increases progressively from thewarm end to the cold end of the refrigerator.

Still another disadvantage of the single casing type refrigerator isthat the top flange at the warm end must be extremely tight to maintainthe required extreme vacuum. Because of practical limitations on theleak tightness of this flange, it is necessary to vacuum pump the innerworking space continually.

Another type of prior art liquid helium temperature refrigerator employsa double-walled vacuum insulated container, with a helium atmosphereinside the inner vessel, and a liquid nitrogen-cooled radiation shieldsurrounding the cold components. However, apparently on the assumptionthat heat transfer losses by gaseous con vection would be prohibitivelyhigh, the inner vessel is filled with powder insulation in which thecold refrigerator components are packed. This prevents maintaining anopen, free access region for use of the inner vessel as a working volumeinto which various experimental heat sources can be readily introduced.That is, the heat source is located at the bottom cold end of therefrigerator, and the powder insulation must be carefully packed aroundthe heat source as well as the superimposed cold assembly before the topflange is closed. Moreover the liquid nitrogen-cooled radiation shieldis expensive, and diflicult to support and operate.

A further limitation of each of these prior art helium fluidrefrigerators is that the cold components must be supported solely byvertical suspension from the warm end flange of the container. In thesingle casing type, lateral stabilization means between the coldcomponents and the casing inner wall cannot be tolerated because of theresultant large heat inleak from the ambient temperature wall. In thedouble-walled, liquid nitrogen- 3,366,955 Patented Jan. 2, 1968 cooledradiation shield type, lateral stabilization means between the coldcomponents and the inner vessel inner wall are blocked by theintervening radiation shield.

An object of this invention is to provide an improved helium fluidrefrigerator.

Another object is to provide such a refrigerator which does not requirea high vacuum atmosphere within the inner working volume of thecontainer.

Still another object is to provide a refrigerator characterized by lowheat transfer losses due to radiation.

A further object is to provide a refrigerator which does not requirepacking of the cold components in powderous insulation within thecontainer inner working volume.

A still further object is to provide a helium fluid refrigerator whichdoes not need liquid nitrogen-cooled radiation shields.

An additional object is to provide such a refrigerator permitting theuse of lateral stabilization means for the cold components.

Other objects and advantages of the invention will be apparent from theensuing description, the claims and the drawings in which:

FIGURE 1 is a schematic flowsheet of a suitable closed circuit liquidhelium refrigeration-producing circuit with metal thermally conductivemembers for use in the invention.

FIGURE 2 is an elevation view taken in cross-section of a cold componentassembly inside the inner working volume of a double-walled vacuuminsulated casing with the warm components of the liquid heliumtemperature refrigerator shown schematically.

We have found that the aforedescribed objects may be achieved by aparticular combination of elements arranged to interact in a heretoforeunemployed manner.

One essential element is a double-walled container comprising an innervessel as the working volume for receiving the cold components and theheat source, and a surrounding outer casing spaced so as to provide anevacuable space therebetween. This space is preferably filled wiith ahighly eflicient solid thermal insulating material, as for examplealternating layers of low conductive fibrous sheeting separatingintervening layers of radiation resistant material such as highlyreflective sheets. The low conductive layers may for example bepermanently precompacted layers of glass fiber paper as described inMatsch U.S.P. 3,009,600 or elastically compressible webs as described inMatsch U.S.P. 3,009,601. The radiation heat reflecting sheets arepreferably aluminum or copper foil.

This double-walled vacuum thermal insulating system permits atemperature gradient down the inner vessel wall, roughly comparable tothe temperature level of the adjacent cold component in the workingvolume. That is, the inner wall section adjacent to the firstsub-ambient temperature heat exchanger will assume this thermal levelwhereas the inner wall section adjacent to the coldest heat exchangerwill approach the boiling temperature of liquid helium. This temperaturegradient drastically reduces the radiative heat losses and improves theoverall efficiency of the refrigerator. For example, it has beendetermined that the side wall heat inleak to a liquid helium temperaturerefrigerator with alternate layers of glass fiber paper and aluminumfoil in the evacuated space of the doublewalled container is about twomagnitudes less than a single casing straight vacuum insulatedrefrigerator.

The helium fluid refrigeration-producing closed circuit means includes agas compressor at the warm end and cold helium fluid expansion means atthe cold end. Heat extraction means are provided intermediate the warmand cold ends. The double-walled vacuum insu- 3 lated container isarranged and positioned so as to gastightly enclose the sub-ambienttemperature cold components of this circuit.

The existence of the temperature gradient along the inner vessel wallpermits the advantageous use of another important element in thisrefrigerator. At least one metal thermally conducting member is providedwithin the double-walled vacuum insulated container, and transverselyspaced between the warm and cold ends of the helium fluidrefrigerationproducing closed circuit means. This thermally conductivemember is thermally associated with both the heat extraction means and arelatively warmer region of the storage vessel inner wall. In thismanner the thermally conductive member serves as a heat station toreceive heat transferred by solid conduction from warmer regions of thestorage vessel inner Wall, and from warmer portions of the heat sourceas well as to intercept radiative heat from the warmer sections of thecontainer. This heat is in turn transferred to the heat extractor, andprevented from flowing to the coldest portion of the refrigerator.

Multiple metal thermally conducting members may be employed andtransversely spaced between the warm and colds ends of the closedcircuit means. In this event multiple heat extraction means areemployed, the relatively warm thermally conductive member beingthermally associated with both the first heat extractor and a relativelywarmer region of the storage vessel inner Wall which is above thetemperature level of the first heat extractor thermal association.

Similarly the relatively colder heat conducting member is thermallyassociated with the second heat extractor and a relatively colder regionof the storage vessel inner wall. This region is selected to be abovethe temperature level of the second heat extractor thermal association,but of course below the temperature level of the first heat extractorthermal association. This second thermally conductive member also servesas a heat station to receive heat transferred by solid conduction fromwarmer portions of the heat source as well as to intercept heat from theintermediate section of the container and prevent same from reaching thecold end.

Additional heat conducting members may be provided and positioned in asimilar manner between the warm and cold ends of the fluid circuit andthe inner storage vessel inner wall. The number of heat conductivemembers which may be advantageously employed depends to a large extenton the number of heat extractors employed in the helium fluidrefrigeration-producing closed circuit.

Since the heat conducting members may contact the inner storage vesselinner wall without introducing additional heat flux into therefrigerator, they may also be employed as lateral support means for therefrigerator and the heat load, as for example electronic components.This additional support function is not possible with a single-walledcontainer because the wall is at ambient temperature throughout itslength.

Another preferred feature of this novel refrigerator is the provision ofa gaseous helium atmosphere within the inner vessel in the spacesurrounding the cold components. The pressure of this helium maybesub-atmospheric or even atmospheric (14.7 p.s.i.a.). This gaseous heliumenvironment affords several significant advantages. From the heattransfer standpoint, it improves the drawing off of heat from the innervessel wall and from the heat load to the metal thermal heat conductingmembers by the medium of gaseous conduction. Absent this gaseousenvironment, heat transfer would be solely by solid conduction. Anotherimportant and very practical advantage of the gaseous helium environmentis that the gas-tightness requirements of all joints are relaxed, ascompared to the tightness needed when the joints separate a high vacuumfrom the ambient pressure. Moreover it is not necessary to vacuum pumpthe inner vessel working space, eliminating the need for the pump andits attendent power supply.

In a preferred embodiment, the metal thermal heat conducting members aresized to provide a narrow annular space of less than about /8 inchbetween their outer edges and the inner storage vessel inner wall. Oneof the significant discoveries embodied in this invention is that suchan annular space coupled with the specified helium atmosphere providesas effective heat transfer between the cooler metal heat conductingmembers and the inner storage vessel inner wall as does lightmetal-to-metal contact under extreme vacuum. This discovery avoids theneed for providing such metal-to-metal contact, an importantsimplification in manufacturing the present helium refrigerator. It isextremely difficult and expensive to provide the relatively thin innerstorage vessel inner wall with a machining tolerance sufficient toinsure continuous but light metal-to-metal contact with the metal heatconducting members. In marked contrast, the inner storage vessel innerwall of this refrigerator need not be machined after shaping. The metalheat conducting members may be simply forrned by conventional means tobe slightly undersized with respect to this wall, thereby providing theneeded small annular space. The helium gas in the space thus providesthe needed thermal association between the two members.

The ability to design the metal heat conducting members as lateralsupport means without reliance upon metalto-metal contact with the innervessel wall permits the entire cold component assembly to be readilyremovable intact from the double-walled container. This greatlyfacilitates the assembly of the cold components and the container, andalso facilitates the installation and/ or inspection of the variousexperimental devices (heat sources) capable of being operated within therefrigerator.

Referring now more specifically to FIGURE 1, helium is compressed toabout 190 p.s.i.a. in compressor 10, cooled to about 300 K. in an aftercooler (not shown) in discharge conduit 11, and directed throughconnecting conduit 12 to passageway 13 of first heat exchanger 14. Inthe latter the compressed helium gas is partially cooled to about 78 K.and part of this gas, e.g. is diverted through conduit 15 to a firstexternal heat extractor as for example first work expander 16. In thelatter the gas is expanded to about 14.7 p.s.i.a. and thereby furthercooled to about K.

The resulting expanded and further cooled helium gas from expander 16 indischarge conduit 17 is heat exchanged in passageway 18 with arelatively warmer section 19 of the inner storage vessel inner wall andwarmed to about K. The partially warmed expanded helium gas is thenjoined with the recycling vaporized helium at about the same temperaturelevel, as subsequently described in detail.

At the same time, the undiverted part of the compressed partially cooledhelium gas emerging from the cold end of first heat exchanger 14 inconduit 12 is directed to passageway 20 in the warm end of second heatexchanger 21 for further cooling to about 62 K. The emerging furthercooled helium gas at the cold end of second heat exchanger 21 is thendirected to passageway 22 atthe warm end of third heat exchanger 23 andstill further cooled therein to about 19 K. A portion, e.g. 60%, of thiscold helium gas is diverted through branch conduit 24 to the higherpressure side of second work expander heat extractor 25. The divertedgas is thus expanded to about 14.7 p.s.i.a. and simultaneouslyadditionally cooled to about K. This additionally cooled expanded heliumgas is heat exchanged in passageway 26 with a section 27 of the storagevessel inner wall and warmed to about 12 K. Section 27 is relativelycolder than section 19 of the storage vessel inner wall, but relativelywarmer than the additionally cooled expanded helium gas. The resultingpartially warmed expanded helium gas is then joined with the recyclingvaporized helium at about the same temperature level, as subsequentlydescribed in detail.

The undiverted cold helium gas at the cold end of third heat exchanger23 at about 19 K. is directed through conduit 12 to selective adsorbentbed 28, e.g. activated charcoal, where the uncondensed impurities suchas neon or hydrogen are removed. The purified cold helium gas is nextflowed to the warm end of passageway 29 in fourth heat exchanger 30where this gas is additionally cooled to about 12 K. The emerging colderhelium gas is then directed to the warm end of passageway 31 in fifthheat exchanger 32 for cooling to about 55 K.

The resulting still colder helium gas is isenthalpicaily expandedthrough valve 33 to atmospheric pressure (14.7 p.s.i.a.), correspondingto the helium boiling point of 4.2 K. The liquid-vapor mixture passes tocontainer 34, and heat source item 34a to be refrigerated is preferablytightly secured against the outer wall 35 of this container forefficient heat transfer by solid conduction. Alternatively the heatsource item 34a may be immersed directly in the liquid helium withincontainer 34. The liquid helium fraction produced by isenthalpicexpansion (the Joule-Thompson effect) is vaporized by this heat load.

The cold helium vapor emerging from container 34 is recycled through thepreviously described five heat exchangers 14, 21, 23, 3t) and 32, whereits sensible refrigeration is recovered in cooling the higher pressurehelium gas. More specifically, this cold helium vapor is initiallydirected through conduit 36 to the cold end of low pressure passageway37 of fifth heat exchanger 32. This vapor enters the exchanger 32 atabout 4.2 K. and flows in countercurrent relation with the cold heliumgas in passageway 31 and is itself partially warmed to about 12 K. Thispartially warmed helium vapor emerges from the warm end of fifthexchanger 32 into conduit 36 and is joined by the partially warmedfurther expanded helium gas in conduit 24 from second work expander 25.This gas is at about the same temperature as the partially warmed heliumvapor so that mixing losses are avoided and process control issimplified.

The combined partially warmed helium stream at about 12 K. in conduit 38is then fed to the cold end of passageway 33 in fourth heat exchanger30. The stream flows in countercurrent heat exchange relation with thepurified helium gas in thermally associated passageway 29, and emergesfrom the warm end at about 16 K. This further warmed helium stream isnext directed to the cold end of passageway 40 in third heat exchanger23 for still further cooling of the helium gas in thermally associatedpassageway 22. The still further warmed low pressure helium gas emergesfrom the warm end of passageway 4%) at about 60 K., and is joined withthe work expanded and partially warmed helium gas from conduit 15 atabout 60 K. As in the case of the second colder work expanded gas, thejuncture occurs in the heat exchanger train at a point where thepartially warmed work expanded gas temperature approximates that of therecycling low pressure helium gas.

The combined low pressure helium gas enters the cold end of passageway41 in second heat exchanger 21 and serves to further cool the undivertedpartially cooled higher pressure helium gas in thermally associatedconduit 20, emerging from the warm end at about 72 K. As a final heatexchange step, this gas is flowed to the cold end of passageway 42 infirst heat exchanger 14 for partial cooling of the higher pressureaftercooled helium gas in thermally associated passageway 13. The warmedlow pressure helium gas emerges from the warm end of passageway 42 atabout 294 K. and is recycled at about 11.4 p.s.i.a. from conduit 43through joining conduit 44 to the suction side of compressor 10. Surgetank 45 is connected across compressor It} by low pressure conduit 46having pressure regulator 47, and by higher pressure conduit 48 havingpressure regulator 49 therein. These regulators supply helium gas tosurge tank 45 and bleed off helium gas therefrom as necessary tomaintain predetermined pressure levels at the warm end of the circuit.Makeup helium gas is supplied to surge tank 45'from an external sourcethrough conduit 50 having control valve 51 therein.

The refrigerator is controlled by adjusting the speed of the expansionengines 16 and 25, the discharge pressure of compressor 10, and thesetting of isenthalpic expansion valve 33. For a given heat load, thecompressor discharge pressure is normally first set by the expansionvalve 33 setting and the expansion engine speeds are used for refinedadjustments. If the heat load increases, more liquid helium is vaporizedin container 34 and discharged thereby depleting the supply. This liquiddeficiency may be replenished by increasing the fluid flow through valve33 by further opening same. In this event, it is necessary to increasethe speed of expansion engines 16 and 25 to increase the helium flowthrough the high pressure side of the heat exchanger train and producemore refrigeration to balance the increased heat load. To accomplishthis increased flow, the pressure regulator 49 is partially closed. Ifthe heat load decreases, adjustments are made opposite to thosedescribed. The temperature level of the boiling liquid helium incontainer 34 is set by controlling the suction pressure of thecompressor 10', using the pressure regulator 47.

Referring now to FIGURE 2, elements corresponding to those illustratedin FIGURE 1 are identified by the same reference numeral plus 100. Theheat exchanger train comprising units 114, 121, 123, and 132 as well asthe first and second work expanders 116 and 125 and the interconnectingfluid conduits are identical to those described in conjunction withFIGURE 1.

The cold component assembly is positioned within double-walled vacuuminsulated container 152 comprising inner storage vessel 153 andsurrounding outer casing 154 with a vacuum space 155 therebetween. Thisspace 155 is for example filled with composite thermal insulation 156comprising alternate layers of low conductive permanently precompactedglass fiber paper sheets and highly reflective aluminum foil. Theindividual glass fibers have predominating diameter of 0.54175 micronand the paper weighs about 1.6 grams/sq. ft. surface area. The aluminumfoil layers may have a thickness of about 0.25 mil and this compositeinsulation is preferably installed at or near its optimum density ofabout 60 layers/ inch of vacuum space cross-section. Space 155 may bepermanently evacuated by attaching a vacuum pump to valve 157communicating with such space through conduit 158, and reducing thepressure to less than one micron Hg. After closing valve 157 the pump isdisconnected. Hydrogen from possible degassing of the metallic wallsenclosing vacuum space 155 is removed by getter material 159, forexample palladium oxide as described in U.S.P. 3,108,706 to L. C. Matschet al. Gases tending to accumulate in vacuum space 155 having higherboiling points than hydrogen, e.g. moisture, are removed by adsorbentmaterial 160, e.g. crystalline zeolitic molecular sieve, as described inU.S.P. 2,900,800 to P. E. Loveday. This adsorbent material 160' isretained in compartment 161 attached to the colder portion of innervessel 153 and in gas communication with vacuum space 155.

The warm open end of container 152 is covered by flange 162 andgas-tightly sealed with O-ring 163. The preferred gaseous heliumatmosphere within the inner vessel working volume 164 is maintained byan external helium source 165 communicating with such volume by means ofgas conduit 166 having control valve 167 therein. Alternatively thisatmosphere could be provided by bleeding 01f a small quantity of lowpressure helium gas from conduit 143 downstream of first heat exchanger114.

The cold component assembly positioned within inner vessel workingvolume 164 includes not only the heat exchanger train but also firstwork expander 116 and second work expander 125. First expander 116includes piston 165 mechanically coupled by reciprocating rod 166 tocrankshaft 167. The piston-rod assembly extends longitudinally inwardlyfrom the container warm end,

7 and is enclosed in housing 168. First expander inlet and dischargevalves 169 and 178 are mechanically coupled by lift rods 171 and 172respectively to crankshaft 167 and are enclosed in respective housings173 and 174. Second expander 125 is mechanically identical to firstexpander 116.

The expanded and further cooled helium gas in conduit 117 is heatexchanged with sheet metal heat conducting plate member 175 by passagethrough serpentine conduit 118 metallically bonded to the upper surfaceof the plate. The latter is also physically supported by the bond toconduit 118 and extends transversely across the inner vessel workingvolume 164 so as to substantially fill this portion of the volumeinwardly of the plate outer edge 176. Plate 175 is preferably sized sothat its outer edge 176 does not physically touch the relatively warmersection 119 of storage vessel inner wall, but a narrow annular space 177of less than about /8 inch is provided therebetween. As previouslydescribed, the helium gas in working volume 164 provides the necessarythermal association between the plate outer edge 176 and storage vesselinner wall warmer section 119 through the heat transfer mechanism ofgaseous conduction. The plate outer edge 176 is bent substantiallyparallel to the inner vessel wall 153 so as to provide an extendedsurface area for efficient heat transfer by gaseous conduction.

As previously discussed, one of the advantages of the presentrefrigerator is that the entire cold component assembly may be laterallystabilized against the storage vessel inner wall without increasingambient heat inleak. Since the preferred embodiment is provided with anannular space between plate outer edge 176 and vessel inner wall warmersection 119, stabilizing means should not entirely enclose this spacebut instead may be spaced around the perimeter of the plate. Suitablestabilizing means may for example comprise multiple pads 178 bonded toplate outer edge 176, formed of a plastic such aspolytetrafluoroethylene and sized to provide light physical contact withthe storage vessel inner wall.

The expanded and still further cooled helium gas in conduit 124 is heatexchanged with colder sheet metal heat conducting plate member 179 bypassage through serpentine conduit 126 metal'lically bonded to the uppersurface of the plate, and in the same manner as warmer sheet metal heatconducting plate member 175. Colder plate 179 is supported in the samemanner as warm plate 175 and also preferably sized to provide a narrowannular space 180 of :less than inch between its outer edge 181 and thestorage vessel inner wall colder section 127. Multiple stabilizing pads182 are spaced around the perimeter of colder plate 179 in annular space180 in a manner analogous to pads 178.

If a metal-to-metal contact between the storage vessel inner wall 153and the plate outer edges 176 and 181 is desired for improved thermalassociation, such contact may be provided by resilient metallic memberssuch as springs or brushes distributed around the plate perimeters.

Heat load 134a is preferably attached to the lower surface 135 of liquidhelium container 134 by for example mechanical means, and is cooled byheat extraction through the metal-.to-metal interface and into theboiling liquid. In all uses of the helium refrigerator for cooling heatloads, there must be means for communication between the load and theambient surroundings. For example, if the heat load is a superconductingmagnet, the communication means may be electric power conductors. If theheat load is a maser crystal, the communication means may be a waveguide and wires for transmitting and receiving signals. Communicationmeans 183 is illustrated as a metal duct which may comprise a wave guideor may contain electric wires preferably secured to the duct bythermally conductive retainers. Heat source communicating means 183extends from the cold end to the warm end of the refrigerator andthrough sealing flange 162. Accordingly it constitutes a source ofambient heat inleak and is thermally associated with metal heatconducting members and 179. As illustrated openings are provided inmembers 175 and 179 for extending heat source communicating means 183therethrough, and the thermal association is enhanced by bending theopening edges 184 and 185 parallel to and in metal-to-metal contact withsuch means 183. Thus, the heat inleak from the heat source communicatingmeans 183 is transferred by metal conducting members 175 and 179 to theoutfiowing expanded low pressure helium gas.

It will be appreciated that any heat inleak passing the colder platemember 179 must be absorbed by the cold helium fluid in container 134.Accordingly, this container does not serve as a heat station in the samemanner as warmer and colder plate members 175 and 176, because thepurpose of these stations is to prevent heat from reaching the heliumfluid at the coldest level of the refrigerator. However, it preferablyextends transversely across the inner vessel working volume in a mannersimilar to these plates and in the form of a thin flangeshaped vessel.

The entire cold component assembly is preferably secured to the warm endflange 162 so that the assembly may be removed from the inner vessel bydisconnecting the flange. Accordingly it is preferred to stabilize theliquid container against the storage vessel inner wall in the samemanner as plates 175 and 179. That is, narrow annular space 186separates the liquid helium container outer edge and storage vesselinner wall 153, and multiple pads 187 are positioned in this spacearound the container perimeter. These pads 187 provide the desiredlateral stabilization of the cold componentsheat load assembly at therefrigerator cold end.

A power absorber as for example rotary electric gencrator 188 is drivenby crankshaft 167. The generated current is extracted through wires 189leading to controller 190 which may contain for example a variableresistance for dissipating the energy. Varying the resistive loadattached to generator 188 affords a convenient means for controlling itsspeed of rotation and hence for controlling the reciprocating speed offirst work expander 116. Any external power required for operation ofcontroller 190 is provided through electrical power connections 191.

Crankcase and power absorber enclosure 192 contains partition 193separating the aforedescribed mechanicalelectrical assembly foroperating first work expander 116 from a similar mechanical electricalassembly for operating second work expander 125.

The use of double-walled, vacuum insulated container whose vacuum space155 is hermetically separated from working volume 164, permits anemergency mode of operating heat load 134a not otherwise possible. Inthe event of mechanical breakdown of the refrigeration-producing closedcircuit means, the operation or test of heat load 134a need not beinterrupted, but may be continued by direct admission of a liquid heliuminto the working volume from an external source. Such admission ofliquid helium may be accomplished by means similar to items 165, 166 and167 for introducing gaseous helium, except that the source willnecessarily be a well-insulated vessel for storing and dispensing thecold liquid. Liquid helium introduced into volume 164 will collect inthe bottom of inner vessel 153, and heat .from heat source 134a will beeffectively transferred across narrow annular space 186 into the innervessel Wall and thence into the boiling liquid.

Although preferred embodiments of this invention have been described indetail, it should be recognized that certain modifications may be madeand certain elements may be deleted, all within its spirit.

For example, the FIGURES 1 and 2 embodiments are specifically directedto a liquid helium refrigerator, but

the invention is equally suitable for providing gaseous heliumrefrigeration at temperatures below about 20 K. In this event, two heatextractors may not be needed to achieve the desired temperature and oneheat extractor may he adequate. Accordingly, only one metal thermallyconducting member would be employed, and thermally associated with thesingle heat extractor.

In a gaseous helium refrigerator it is not necessary to use anisenthalpic expansion valve at the cold end of the closed circuit, butinstead pass the entire helium gas stream through a work expander heatextractor at such cold end. The resulting cold helium gas may be heatexchanged with a heat source within the inner vessel Working volume, oralternatively the cold gas may be directed through a thermally insulatedconduit to the outside of the refrigerator for heat exchange with theheat source, and thereafter returned to the refrigerator. In the latterembodiment, the thermally insulated conduit is preferably passed throughthe metal thermally conducting members in thermal association therewith.Similarly, helium liquid may be transferred through a thermallyinsulated conduit to the refrigerator exterior for heat exchange withthe heat source, and recycled as gas.

Although isentropic expansion engines, i.e., work expanders, are thepreferred heat extractors in the present refrigerator, other Well-knowntypes of heat extractors may be employed as for example an externalsource of liquid nitrogen, liquid air, or liquid hydrogen. In such anembodiment the higher pressure helium gas discharged from a warmer heatexchanger may be passed through a coil immersed in the liquid nitrogencontainer within the inner vessel working volume, and dischargedtherefrom at a colder temperature for further cooling in the remainingunits of the heat exchanger train. The liquid nitrogen will of courseboil by virtue of this heat transfer, and the resulting vapor may bedischarged in a conduit extending through the working volume and thecontainer wall or top flange for release to the atmosphere. The nitrogenboil-off may be replaced by a second conduit connected to an externalliquid nitrogen source and also extending through the container wall ortop flange to the liquid nitrogen container in the working volume. Themetal thermally conducting member may be bonded to the liquid nitrogencontainer outer wall to effect the necessary thermal association andheat transfer.

Still another suitable though less effective heat extractor means is thethrottle expansion of higher pressure cold gaseous helium at atemperature below about 40 K. to a lower pressure. Each throttleexpansion through a suitable valve will additionally cool the lowerpressure gas, which in turn may be heat exchanged with the metalthermally conductive member. This heat exchange may for example beobtained by bonding the throttle expansion discharge conduit to thethermally conductive member.

What is claimed is:

1. A helium refrigerator comprising:

(a) helium fluid refrigeration-producing closed circuit means having aWarm end and including a gas compressor at the warm end as anabove-ambient temperature component, and including as sub-ambienttemperature cold components: (1) cold helium fluid expansion means atthe cold end, (2) heat extraction means intermediate the warm and coldends, and (3) means for heat exchanging compressed warmer helium gas andcolder lower pressure helium gas intermediate the warm and cold ends;

(b) a heat source and means for heat exchanging the helium fluiddischarged from said cold helium fluid expansion means (a) (1) with saidheat source;

(c) a double-walled vacuum insulated container comprising an innerstorage vessel and a surrounding outer casing with a vacuum spacetherebetween, and a removable cover flange arranged and positioned so asto gas-tightly enclose and suspend the subambient temperature coldcomponents of said helium fluid refrigeration-producing closed circuitmeans (a) from said cover flange; and

(d) a metal thermally conducting member within said double-walled vacuuminsulated container transversely spaced and suspended between the warmand cold ends of the said helium fluid refrigerationproducing closedcircuit means (a), being thermally associated with said heat extractionmeans and also thermally associated with a relatively warmer region ofthe storage vessel inner Wall so as to receive heat from such region forsolid conductive transfer to the heat extractor means.

2. A helium refrigerator according to claim 1 in which said heatextraction means is a work expander.

3. A helium refrigerator according to claim 1 in which said heatextraction means is an external supply of liquid nitrogen.

4. A helium refrigerator according to claim 1 in which said metalthermally conducting member is sized to provide a narrow annular spaceof less than about 4; inch between its outer edge and the storage vesselinner wall.

5. A helium refrigerator according to claim 4 in which said metalthermally conducting member is additionally shaped to substantially fillthe transverse space within said inner storage vessel inwardly of saidnarrow annular space and surrounding the outer surface of said heliumfluid refrigeration-producing closed circuit means (a) being positionedsubstantially normal to and extending through said metal thermallyconducting member.

6. A helium refrigerator according to claim 1 in which said metalthermally conducting member is arranged and positioned to laterallysupport said helium fluid refrigeration-producing closed circuit meanswithin said storage vessel.

7. A helium refrigerator according to claim 1 in which said heatextractor means comprises a work expander and low pressure cold heliumdischarge conduit means, and said metal thermally conductive member isthermally associated with said conduit means.

8. A helium refrigerator comprising:

(a) helium fluid refrigeration-producing closed circuit means having awarm end and a cold end and including a gas compressor at the warm endas an above-ambient temperature component, and including a sub-ambienttemperature cold components: (1) an isenthalpic expansion valve and (2)a liquid helium receiving container at the cold end, (3) a first heatextractor at a relatively warmer intermediate level, (4) a second workexpander heat extractor at a relatively colder thermal level, and (5)means for heat exchanging compressed warmer helium gas and colder lowerpressure helium vapor;

(b) a heat source thermally associated with said liquid helium receivingcontainer;

(c) a double-walled vacuum insulated container comprising an innerstorage vessel and a surrounding outer casing with a vacuum spacetherebetween and a removable cover flange arranged and positioned so asto gas-tightly enclose and suspend the sub-ambient temperature coldcomponents of said helium fluid refrigeration-producing closed circuitmeans (a) from said cover flange;

(d) at least two metal thermally conducting members within said=double-walled vacuum insulated container each transversely spaced andsuspended between the warm and cold ends of said helium fluidrefrigeration-producing closed circuit means (a), the relatively Warmthermally conducting member being thermally associated with said firstheat extractor and also thermally associated with a relatively warmerregion of the storage vessel inner wall which is above the temperaturelevel of the first heat extractor thermal association so as to receiveheat from said relatively warmer region for 1 1 l .2 solid conductivetransfer to said first heat extractor, References Cited and therelatively colder heat conducting member UNITED STATES PATENTS beingthermally associated with said second Work expander heat extractor andalso thermally asso- 2966O34 12/1960 Glfiord. T 62 6 ciated with arelatively colder region of the storage 5 3092976, 6/1963Hashemlfrafreshl 62 335 vessel i er wall which is above the temperature311501J 12/1963 Hogan 62 6 lev l of the second heat extractor thermalassocia- 3122044 2/1964 Abefle 62 259 ti o a to receive heat from saidrelatively colder 3125863 3/1964 9 62-332 region for solid conductivetransfer to said second 31286O5 4/1964 Malaker 62 -6 work expander heatextractor; and 10 g2 67 (e) means for providing a gaseous helium envir0n3220701 11/1965 6 ment within said inner storage vessel and in the g1/1967 5:; mg 5 2 space surrounding the closed circuit means (a) andsaid metal thermal conducting members (d). OTHER REFERENCES 9. A heliumrefrigerator according to claim 1 in which 15 alternate layers of glassfiber paper sheets and reflective metal foil are provided as compositethermal insulation material in the vacuum space of said double-Walledvacuum insulated container WILLIAM J. WYE, Primary Examiner.

Collins and Cannaday: Expansion Machines for Low Temperature Processes,pp. 4064, 108112, Oxford University Press, 1958.

1. A HELIUM REFRIGERATOR COMPRISING: (A) HELIUM FLUIDREFRIGERATION-PRODUCING CLOSED CIRCUIT MEANS HAVING A WARN END ANDINCLUDING A GAS COMPRESSOR AT THE WARM END AS AN ABOVE-AMBIENTTEMPERATURE COMPONENT, AND INCLUDING AS SUB-AMBIENT TEMPERATURE COLDCOMPONENTS: (1) COLD HELIUM FLUID EXPANSION MEANS AT THE COLD END, (2)HEAT EXTRACTION MEANS INTERMEDIATE THE WARM AND COLD ENDS, AND (3) MEANSFOR HEAT EXCHANGING COMPRESSED WARMER HELIUM GAS AND COLDER LOWERPRESSURE HELIUM GAS INTERMEDIATE THE WARM AND COLD ENDS; (B) A HEATSOURCE AND MEANS FOR HEAT EXCHANGING THE HELIUM FLUID DISCHARGED FROMSAID COLD HELIUM FLUID EXPANSION MEANS (A) (1) WITH SAID HEAT SOURCE;(C) A DOUBLE-WALLED VACUUM INSULATED CONTAINER COMPRISING AN INNERSTORAGE VESSEL AND A SURROUNDING OUTER CASING WITH A VACUUM SPACETHEREBETWEEN, AND A REMOVABLE COVER FLANGE ARRANGED AND POSITIONED SO ASTO GAS-TIGHTLY ENCLOSE AND SUSPEND THE SUBAMBIENT TEMPERATURE COLDCOMPONENTS OF SAID HELIUM FLUID REFRIGERATION-PRODUCING CLOSED CIRCUITMEANS (A) FROM SAID COVER FLANGE; AND (D) A METAL THERMALLY CONDUCTINGMEMBER WITHIN SAID DOUBLE-WALLED VACUUM INSULATED CONTAINER TRANSVERSELYSPACED AND SUSPENDED BETWEEN THE WARM AND COLD ENDS OF THE SAID HELIUMFLUID REFRIGERATIONPRODUCING CLOSED CIRCUIT MEANS (A), BEING THERMALLYASSOCIATED WITH SAID HEAT EXTRACTION MEANS AND ALSO THERMALLY ASSOCIATEDWITH A RELATIVELY WARMER REGION OF THE STORAGE VESSEL INNER WALL SO ASTO RECEIVE HEAT FROM SUCH REGION FOR SOLID CONDUCTIVE TRANSFER TO THEHEAT EXTRACTOR MEANS.